1. Field of the Invention
This invention relates to a method of producing a coated high voltage insulator. The surface of the insulator is a cured one part, room temperature curable silicone composition.
2. Description of the Prior Art
High voltage electrical insulators have historically been constructed of glass and porcelain. These materials of construction allowed the manufacture of insulators such as the familiar post insulators and cap and pin suspension insulators. Under normal conditions, these insulators gave a long service life in spite of exposure to weathering, mild contamination, and electrical stress and leakage currents. Under conditions of severe contamination, frequent cleaning or coating with silicone grease-like compounds was found necessary to prevent excessive leakage currents. With the use of higher voltages for transmission, the size and weight of this type of insulator has become excessive. At very high voltages, greater than 375 KV for instance, the creepage distance requirements dictate an insulator of such a length that the mechanical strength of ceramic cap and pin insulators is inadequate. Composite insulators are now being evaluated using organic and inorganic resins and elastomers as a substitute in locations where the glass and porcelain insulators are not suitable. Failure of composite insulators has occurred in some areas having severe contamination.
In the use and testing of composite insulators, it has become evident that the materials of construction and the design of the insulator must be selected to satisfy the unique combination of physical and electrical requirements imposed on the insulators. Many different methods of producing and improving composite insulators have been proposed and developed.
A weatherable insulator taught by Talcott in U.S. Pat. No. 3,511,698, issued May 12, 1970, comprises a rigid, cured thermosetting resin base member and a surface coating of at least 0.25 mm thickness of cured, organopolysiloxane elastomer. The elastomer is a stock based upon an organopolysiloxane containing SiH groups or Si bonded alkenyl radicals or both or a combination of organopolysiloxane and crosslinker where one type of functional group is in the base polymer and the other in the crosslinking agent. A platinum material is used to catalyze the cure to an elastomer. It is desirable that the elastomer stock contain from 5 to 15 weight percent of hydrated aluminum oxide to improve the track resistance.
In a paper given at the 10th Electrical Insulation Conference, held Sept. 20 to 23, 1971, titled "Self-Priming Silicone Elastomeric Coatings for High Voltage Insulator Bodies," I taught that a silicone composition containing Me(H)SiO--and (Me).sub.2 SiO--organic substituents in the siloxane structure gave the best results, when tested for hydrophobicity, both before and after corona exposure, and for arc-track resistance, as a coating for insulators.
Penneck teaches in Canadian patent No. 889,898, issued Jan. 4, 1972, that an improved high voltage insulating material is a mixture of polymer and filler which is termed "anti-tracking filler system". This anti-tracking filler system comprises a hydrate of alumina having a specific surface area of at least 2 m.sup.2 /g and an oxide, mixed oxide, or mixture of oxides containing at least one element from the transition elements, the lanthanide series, or the actinide series. Among his polymeric materials taught as useful are polydimethylsiloxane, dimethylsiloxane/methylvinylsiloxane copolymers, fluorosilicones, and carborane siloxanes. In his examples, he shows curing a dimethylsilicone elastomer with an organic peroxide.
In U.S. Pat. No. 3,965,065, issued June 22, 1976, Elliott teaches a method of improving the electrical properties of organopolysiloxane elastomers by heating a mixture of an organopolysiloxane convertable to the solid elastic state and aluminum hydrate for at least 30 minutes at a temperature above 100.degree. C. to provide a composition which can be cured to an elastomer having improved electrical properties. He teaches the elastomer forming compositions of his invention may be converted to the elastic state by any suitable curing technique, including so-called cold curing mechanisms wherein the convertible organopolysiloxane contains functional radicals, e.g. hydroxyl, oximo, or acyloxy and curing takes place at normal or slightly elevated temperatures with or without the addition of a crosslinking agent and a curing catalyst.
In U.S. Pat. No. 4,189,392, issued Feb. 19, 1980, Penneck teaches an electrical insulating material suitable for high voltage applications comprising a polymeric material having incorporated therein an anti-tracking material such as an inorganic metal oxide, hydroxide, or salt, including hydrates of alumina and an erosion inhibiting, hydrophobic, non-ionic, fluorine substituted compound. Silicone polymers are included in his list of suitable polymers.
In U.S. Pat. No. 4,011,168, issued Mar. 8, 1977, Uhlman teaches a composition useful as an electrical coating to protect transmission insulators. The composition consists essentially of a siloxane fluid and from 50 to 80 percent by weight of a mineral filler selected from the group consisting of clays, ground quartz, gypsum, silica, and aluminum oxide. Finely divided hydrated aluminum oxide is most preferred. The composition is a non-curing product of a grease-like consistency.
In U.S. Pat. No. 4,177,322, issued Dec. 4, 1979, Homan et al. teach a specific polydiorganosiloxane fluid which is coated on the surface of devices such as electrical transmission line suspension insulators to improve their resistance to the development of excessive leakage currents.